As discussed in U.S. Pat. No. 3,710,890, the perceivable noise that emanates from a jet engine can be classified according to its source. One source of noise, called jet noise, is attributed to a shearing action of gas molecules as they are discharged at high pressure, and thus high velocities, into the surrounding atmosphere. The intensity of the resulting noise is proportional to the velocity of the discharged gases. Because of the extremely high velocity of gases in the primary flow, jet noise from this source is one of the most objectionable noise components. An effective technique for suppressing jet noise is to equip the ducted-fan, turbojet engine with a mixing device, in which the fan and primary flows are mixed prior to their discharge into the atmosphere in order to convert the high-velocity primary flow and the low-velocity fan flow into a relatively homogeneous mixed flow of intermediate velocity. Neglecting losses introduced by the mixing structure of such a device, the thrust produced by the mixed flow is theoretically greater than the total thrust produced by the sum of the primary flow plus the fan flow.
However, in actuality, the losses introduced by the mixing device cannot be ignored and indeed such losses constitute a practical limitation on the use of his technique to reduce noise. In particular, the mixer equipped engine incurs increases in pressure losses in two ways. First, the mixer duct which is used in the mixing device increases the structural surface area that the primary and fan flows encounter. This in turn means an increase in the surface friction drag to which the primary and fan flows are subjected. The friction drag robs momentum from the gas molecules before they reach the discharge opening of the nozzle and thus reduces thrust. Secondly, the mixing device can create secondary flows e.g., swirling motion, and/or flow separations, thereby causing further loss of pressure and hence thrust.
Also, it has been the experience of engine designers that existing designs for mixing devices cannot be simply modified to enhance the degree of mixing (and thus maximize the noise suppression) without recognizing and accounting for possible adverse effects on the thrust performance. For example, the mixing of the primary and fan flows can normally be enhanced by extending the nozzle length to allow greater blending of the turbine gases and fan air after they pass through the mixing device and before they are discharged from the aft terminus of the nozzle. However, such an increase in nozzle length is accompanied by greater internal surface area of the nozzle and thus by more pressure loss due to friction drag. Additionally, attempts have been made to improve the thoroughness of the mixing by various forms of baffling and other internal mixer structures, but it has usually been found that although the mixing has been improved, there has been such a disruption of the rearward flow of the gases that unacceptably high pressure losses result.
Accordingly, it is an object of the invention to enhance the degree of mixing of primary and fan flows in a mixer equipped, ducted-fan, turbojet engine in order to reduce jet noise while at the same time preserving most of the gains in thrust that are expected, in theory, to result from an idealized mixer operation.